This application is based on an application No. 2001-138155 filed in Japan, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an integrated circuit device, and especially relates to inter-device isolation.
2. Related Art
FIGS. 1A and 1B are schematic representations of an integrated circuit device which is a related art of the present invention. FIG. 1A is a plan view, whereas FIG. 1B is a sectional view. This integrated circuit device is roughly made up of a semiconductor substrate 901, a first circuit 907, a second circuit 908, and connector terminals 906. The semiconductor substrate 901 is formed from Si. The first circuit 907 includes semiconductor devices formed in the semiconductor substrate 901 and an insulator film and wires formed on the semiconductor substrate 901 (not illustrated). The second circuit 908 is provided at a different location from the first circuit 907. The connector terminals 906 perform signal input/output with outside the integrated circuit device.
Also, an isolation region 902 is provided around the first circuit 907, to prevent a signal from leaking between the first circuit 907 and the second circuit 908 through the semiconductor substrate 901 which has conductivity.
This isolation region 902 is formed by etching a trench in the semiconductor substrate 901 and filling the trench with an insulator such as silicon dioxide. In this way, a part of the semiconductor substrate 901 which belongs to the first circuit 907 and a part of the semiconductor substrate 901 which belongs to the second circuit 908 are isolated from each other.
However, the isolation region 902, provided between the part of the semiconductor substrate 901 which belongs to the first circuit 907 and the part of the semiconductor substrate 901 which belongs to the second circuit 908, forms the electrical equivalent of a capacitor. Hence the isolation region 902 cannot sufficiently suppress signal interference in a high-frequency range.
FIG. 2 shows an equivalent circuit of the integrated circuit device shown in FIGS. 1A and 1B. The principle of signal interference is explained below, by referring to FIG. 2.
Each of the wires and semiconductor devices (not illustrated) included in the first and second circuits 907 and 908 is coupled to the semiconductor substrate 901, through the capacitance of the insulator film (not illustrated) typically formed between the wire and the semiconductor substrate 901 and the junction capacitance between the semiconductor device and the semiconductor substrate 901. These can be represented by capacitances 920, a part 921a of the semiconductor substrate 901 which belongs to the first circuit 907, and a part 921b of the semiconductor substrate 901 which belongs to the second circuit 908.
The part 921a and the part 921b are opposed to each other with the isolation region 902 interposed in between. This isolation region 902 is made up of the insulator embedded in the trench. Accordingly, it can be said that the part 921a and the part 921b are connected to each other via an isolation capacitance 922. A signal having a high-frequency component passes through the isolation capacitance 922, so that interference occurs between the first circuit 907 and the second circuit 908 due to the signal leakage. This reduces the performance of the integrated circuit device. For example, if a noise signal having a high-frequency component is caused by a high-amplitude signal generated from a logic circuit or a local oscillator and leaks to another circuit that handles weaker signals, such as a radio signal receiving circuit, the performance of the receiving circuit may be markedly reduced. In particular, in the case of a direct conversion architecture which has been under study in recent years, a high-frequency oscillator needs to be used. This makes it impossible to ignore the effects of noise generated from such an oscillator. Thus, there are difficulties in integrating all necessary functions of a radio transmission/reception device, including receivers, logic circuits, and local oscillators, into a single integrated circuit device.
The present invention has an object of providing an integrated circuit device that can suppress undesired effects caused by leakage of noise signals between a plurality of circuits.
The stated object can be achieved by an integrated circuit device including: a first circuit which is formed on a semiconductor substrate; a second circuit which is formed on the semiconductor substrate at a location different from the first circuit; an isolation region which has a higher conductivity than the semiconductor substrate, and is formed in the semiconductor substrate between the first circuit and the second circuit; and a low impedance voltage output circuit which provides a fixed voltage at a low impedance, to the isolation region.
With this construction, even when a noise signal having a high-frequency component occurs, a current caused by the noise signal flows through the isolation region into the low impedance voltage output circuit, so that signal leakage between the first and second circuits is suppressed. This enables different circuits, such as logic circuits and transmission/reception circuits, which handle largely different levels of signals, to be integrated into a single integrated circuit device.